Statement of the Technical Field
The present invention relates generally to Integrated Circuits (“ICs”). More particularly, the present invention relates to passive Radio Frequency Identification (“RFID”) tags including ICs using sub-threshold technology.
Description of the Related Art
RFID technology has conventionally been used to transfer data for purposes of automatically identifying and tracking RFID tags attached to objects. In this regard, the RFID tags have information stored in a data store thereof, such as a unique identifier. During operation, the RFID tags transmit a response to a signal received from an RFID reader by backscattering the received signal. At a minimum, the response signal includes the unique identifier.
Some RFID tags are passive. Passive RFID tags get their operating power by harvesting energy from the signal received from an RFID reader. More specifically, the RFID tags are powered by electromagnetic induction from magnetic fields produced near a reader thereof by a local radiator. Electromagnetic induction is the production of an electromotive force across a conductor when it is exposed to a varying magnetic field. Passive RFID tags have many advantages such as low cost, light weight, small form factor and long operational life. However, passive RFID tags have a limited read range since only a few microwatts can be harvested from the received signal.
Gains in the read range have occurred because the power needed to run the passive RFID tags has decreased over time. Additionally, the passive RFID tags' sensitivity to incoming signals has gone down over time. However, in general, the small antennas of the passive RFID tags limit the amount of power supplied thereto. In effect, the read range of the passive RFID tags is still limited to 2-10 meters from the RFID reader.